This invention relates to shielded radio frequency transmission cable, and particularly to improvements in apparatus, techniques, and materials for the type of shielded radio frequency transmission cable disclosed in Smith, U.S. Pat. No. 4,375,920.
The specification of the Smith patent describes a well shielded cable having at least one center conductor, a dielectric surrounding the center conductor to establish a primary transmission path, and at least two metallic sheaths separated to provide a high series impedance. The space between the sheaths is occupied by an extruded dielectric interlayer and the materials, configuration, and sizes of the sheaths and the dielectric interlayer are selected to establish a high propagation function (propagation constant) for the second transmission path that results between the sheaths. In particular, Smith's specification discloses attaining the desired high propagation function by special features of the sheaths and/or the dielectric, i.e., by (1) proper selection of the resistance and inductance of at least one of the sheaths and the electrical properties of the dielectric, (2) using a dielectric interlayer material characterized as electrically poor by nature of the material or by the loading of the dielectric with lossy pigment, (3) using a laminated interlayer of electrically good and electrically poor dielectrics, or (4) using electrically poor conductors for the sheaths and an electrically good dielectric material.
Some other types of cable have a shield consisting of two layers of cigarette-wrapped aluminum and plastic laminate tape, with a layer of braided aluminum or copper round wires between the laminate tape layers.
It is also known in various types of cable to use an uninsulated metallic drain wire (in electrical contact with the conductive surface of a metallic sheath) to drain off excess current from a sheath, to ground a sheath (especially where the sheath itself has limited current carrying capacity, e.g., where it comprises a thin metallic film), or to provide an easier means to terminate a sheath. Longitudinal drain wires inhibit flexure of the cable and thus preferably are helically wound around the sheath with a very long lay to minimize the amount of wire required while allowing the desired flexibility, as disclosed in Timmons, U.S. Pat. No. 3,032,604. The drain wire may be either exposed, embedded in the dielectric material surrounding the center conductor, or embedded in the jacket material surrounding the metallic sheath. In twisted pair cables having a drain wire inside a coaxial sheath, the lay of the drain wire preferably matches the twisted pair for ease of manufacturing, as shown in U.S. Pat. Nos. 4,096,346, Stine et al.; and 4,041,237, Stine. Drain wires are also disclosed in U.S. Pat. Nos. 4,157,518, McCarthy; 4,323,721, Kincaid, et al.; 4,327,246, Kincaid; and 4,376,920, Smith.
U.S. Pat. No. 3,666,877 Arnaudin, Jr., et al.; disclose embedding a drain conductor in a semi-conducting cable jacket to electrically reinforce the jacket.
In the case of shielded radio frequency radiating cables, a desired degradation of the cable shielding which improves the desired radiation, can be obtained by a wire wound helically onto the outer surface of, and preferably in electrical contact with, a radiating metallic sheath, and may be embedded in the cable jacket. U.S. Pat. No. 3.949,329, Martin; discloses winding the wire on a braid to increase the inductance seen by the leakage wave propagating on the outer surface of the metallic sheath. This increase in inductance is desired to control the velocity of the leakage wave thereby enhancing the performance of the radiating cable. As shown in U.S. Pat. Nos. 3,870,977, peoples et al.; and 4,339,733, Smith; wires may be used on the inner surface of a leaky metallic coaxial sheath to enhance the radiation from apertures in the sheath. These known enhancements of radiation from a radiating cable reflect a desired degradation of cable shielding.
Cables are often mechanically supported by so-called messengers, i.e., tension bearing metal wires embedded in the jacket.